He does need to be getting acclimatised to the outside world. Carrying him to/at different places to get him used to different sights, sounds and smells, plus different people and dogs and other animals is a good idea.
Start off watching from a distance, if pup looks nervous, go back a step, put more distance between you and whatever they’re nervous of and try again. Plenty of treats to make good associations help as well. We tend find a slightly remote bench and sit there with a pot of treats, watching the world go by. It’s a bonding experience, as well as useful socialisation. After gradually increasing proximity according to how the puppy reacts, we will find a bench in our small town centre and repeat the process there. We also drive to out of town shopping centres and supermarkets, park up, lift the boot and sit with them dropping treats as they watch cars, buses, people, shopping trolleys etc.
Don’t let people just come up and fuss/coo over him while he’s being held. This can be really scary for some pups, as they can’t escape and feel trapped and intimidated. Let him see people from a distance, gradually reducing the distance at the pace your particular pup is happy with and if he seems keen and interested to interact, just get people to say a calm hello. Visiting other people’s houses and having visitors, where pup can be off-lead and choose his own pace for approaching to greet is preferable.
Socialisation is so much more than just learning to be around other dogs and people, it’s sights/sounds/smells/different surfaces/handling and a lot more besides.
Dr Sophia Yin’s website has an excellent Socialisation Checklist. It’s very comprehensive and you may feel not all of it applies to your pup and lifestyle, but it’s still a really helpful guide.
I would also recommend reading Easy Peasy Puppy Squeezy, by Steve Mann and The Happy Puppy Handbook by Pippa Mattinson, both are quick easy reads and you get get them on Kindle.
The most important thing is to learn to read your pup and work to his pace, only moving on when he’s happy to. Yes, there are socialisation windows, but moving too fast and bombarding a young pup with experiences they can’t cope with can be as detrimental as not socialising, so it’s important it’s done on an individual basis.